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Overdrive transmissions can be automatic or manually-shifted gearboxes that offer several benefits over a typical non-overdrive unit. Operating at a decreased gear ratio, overdrive transmissions actually turn at fewer revolutions than the vehicle's engine is turning, saving fuel, wear on engine components and generating reduced emissions in most cases. The majority of vehicles found on the world's roads are equipped with overdrive transmissions in one design or another. For the factory-equipped overdrive transmission, the overdrive is commonly used as the standard drive gear with a disengagement or on/off switch mounted on the gear-selector handle. On other vehicles equipped with aftermarket bolt-on overdrive units, the transmission is commonly controlled via a switch or handle mounted under the vehicle's dashboard.
With a non-over-drive transmission, the final gear ratio is commonly 1:1, or one turn of the drive shaft for every complete turn of the engine's crankshaft. This allows for an amount of power from the vehicle that still retains driving capability on long trips. Any lower gear ratio would result in the engine operating at very high revolutions per minute (RPMs). In overdrive transmissions, the final gear ratio is commonly 25-percent lower, with some final drive ratios being 2.67:1, or nearly three turns of the drive shaft for every revolution of the engine's crankshaft. This is how overdrive transmissions can produce enormous fuel savings when compared to other transmissions.
The advantage in being able to switch overdrive transmissions off is seen when pulling a trailer or hauling a heavy load. In order for the overdrive to function properly, the engine must operate at a very reduced speed. This causes the engine to work outside of its power band or most powerful engine speed. If pulling a heavy load with the overdrive function engaged, the engine will continually shift up and down while it tries to gain power. If allowed to continue in this manner, overdrive transmissions will develop severe heating of the transmission fluid and components and will eventually fail.
The proper final gear ratio for overdrive transmissions is derived by factoring the vehicle's weight, horsepower and other factors such as rear end gearing and power adders. In some vehicles equipped with roots-type blowers or superchargers, installing overdrive transmissions can increase fuel economy by as much as 100 percent. This is due to many factors such as reduced heat from the blower by turning the engine at slower speeds and tailoring the shift points for optimal performance.
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